Fringe projection imaging uses patterned or structured light to obtain surface contour information for structures of various types. In fringe projection imaging, a pattern of lines of an interference fringe or grating is projected toward the surface of an object from a given direction. The projected pattern from the surface is then viewed from another direction as a contour image, taking advantage of triangulation in order to analyze surface information based on the appearance of contour lines. Phase shifting, in which the projected pattern is incrementally spatially shifted for obtaining additional measurements at the new locations, is typically applied as part of fringe projection imaging, used in order to complete the contour mapping of the surface and to increase overall resolution in the contour image.
Fringe projection imaging has been used effectively for surface contour imaging of solid, highly opaque objects and has been used for imaging the surface contours for some portions of the human body and for obtaining detailed data about skin structure. However, a number of technical obstacles have prevented effective use of fringe projection imaging of the tooth. One particular challenge with dental surface imaging relates to tooth translucency. Translucent or semi-translucent materials in general are known to be particularly troublesome for fringe projection imaging. Subsurface scattering in translucent structures can reduce the overall signal-to-noise (S/N) ratio and shift the light intensity, causing inaccurate height data. Another problem relates to high levels of reflection for various tooth surfaces. Highly reflective materials, particularly hollowed reflective structures, can effectively reduce the dynamic range of this type of imaging.
In fringe projection imaging overall, contrast is typically poor, with noise as a significant factor. To improve contrast, many fringe projection imaging systems take measures to reduce the amount of noise in the contour image. In general, for accurate surface geometry measurement using fringe imaging techniques, it is useful to obtain the light that is directly reflected from the surface of a structure under test and to reject light that is reflected from material or structures that lie beneath the surface. This is the approach that is generally recommended for 3D surface scanning of translucent objects. A similar approach must be used for intra-oral imaging.
From an optics perspective, the structure of the tooth itself presents a number of additional challenges for fringe projection imaging. As noted earlier, light penetrating beneath the surface of the tooth tends to undergo significant scattering within the translucent tooth material. Moreover, reflection from opaque features beneath the tooth surface can also occur, adding noise that degrades the sensed signal and thus further complicating the task of tooth surface analysis.
One corrective measure that has been attempted to make fringe projection workable for contour imaging of the tooth is application of a coating that changes the reflective characteristics of the tooth surface itself. Here, to compensate for problems caused by the relative translucence of the tooth, a number of conventional tooth contour imaging systems apply a paint or reflective powder to the tooth surface prior to surface contour imaging. For the purposes of fringe projection imaging, this added step enhances the opacity of the tooth and eliminates or reduces the scattered light effects noted earlier. However, there are drawbacks to this type of approach. The step of applying a coating powder or liquid adds cost and time to the tooth contour imaging process. Because the thickness of the coating layer is often non-uniform over the entire tooth surface, measurement errors readily result. Significantly, the applied coating, while it facilitates contour imaging, can tend to mask other problems with the tooth and can thus reduce the overall amount of information that can be obtained.
Even where a coating or other type of surface conditioning of the tooth is used, however, results can be disappointing due to the pronounced contours of the tooth surface. It can be difficult to provide sufficient amounts of light onto, and sense light reflected back from, all of the tooth surfaces. The different surfaces of the tooth can be oriented at 90 degrees relative to each other, making it difficult to direct enough light for accurately imaging all parts of the tooth.
There have been a number of attempts to adapt structured light surface-profiling techniques to the problems of tooth structure imaging. For example, U.S. Pat. No. 5,372,502 entitled “Optical Probe and Method for the Three-Dimensional Surveying of Teeth” to Massen et al. describes the use of an LCD matrix to form patterns of stripes for projection onto the tooth surface. A similar approach is described in U.S. Patent Application Publication 2007/0086762 entitled “Front End for 3-D Imaging Camera” by O'Keefe et al. U.S. Pat. No. 7,312,924 entitled “Polarizing Multiplexer and Methods for Intra-Oral Scanning” to Trissel describes a method for profiling the tooth surface using triangularization and polarized light, but needing application of a fluorescent coating for operation. Similarly, U.S. Pat. No. 6,885,464 entitled “3-D Camera for Recording Surface Structures, In Particular for Dental Purposes” to Pfeiffer et al. discloses a dental imaging apparatus using triangularization but also requiring the application of an opaque powder to the tooth surface for imaging.
It is known that cracks or fractures in teeth can be difficult to detect, whether using visible light or x-ray imaging. U.S. Pat. No. 4,204,978 to Ibsen et al. discloses tooth crack detection using a composition or solution for detecting the location of normally invisible cracks in a tooth. Such methods require a number of chemicals and require operator training; further, the materials used can stain the tooth. U.S. Pat. No. 6,584,341 to Mandelis et al. describes a photothermal radiometric and luminescence method for locating cracks along the enamel surface. The method in '341 irradiates a portion of a surface of a tooth at an effective wavelength. Both photothermal radiometric signals and luminescence signals are then emitted from the portion of the tooth that has been irradiated. The photothermal radiometric signals and luminescence signals are detected and demodulated into phase and amplitude components. The demodulated phase and amplitude components are compared to luminescence phase and amplitude signals of a reference sample to determine differences between the portion of the tooth and the reference sample. This type of method is fairly costly and complex, making it impractical for widespread use.
It can be appreciated that there would be benefits to a low-cost apparatus and method that not only provides accurate surface contour imaging of the tooth, without the need for applying an added coating or other conditioning of the tooth surface for this purpose, but also provides crack detection without other equipment or light source.